This invention pertains to a method of increasing tissue accumulation and retention of radiolabeled compounds (radioligands), thus improving their therapeutic and diagnostic value.
Radiolabelled compounds are used for both tumor detection and tumor therapy. Many tumor cells have a higher density of cell receptors for various circulating compounds than do non-tumor cells; e.g., endocrine tumors show a high density of cell surface receptors for somatostatin, and brain gliomas show a high density of receptors for epidermal growth factor. Thus a radiolabeled compound that binds to these cellular receptors preferentially binds to the tumor cells. Additionally, angiogenesis, the formation of new blood vessels from established microvasculature, is a critical process for tumor growth. Primary tumors and metastases will not grow beyond 2 mm in diameter without an enhanced vascular supply. Angiogenic cells also have a higher density of cell receptors for various circulating compounds than do non-angiogenic vascular tissue; e.g., receptors for both somatostatin and vascular endothelial growth factor are higher in angiogenic tissue. Thus a tumor can also be detected by radiolabeled compounds binding to the angiogenic cells that are closely associated with the tumor cells.
An ideal tumor imaging agent would maximize the radioactivity at the target cells, and minimize the background signal, resulting in a well-defined image of the tumor foci. For example, 111In-DTPA-D-Phe-1-octreotide and 123I-vasoactive intestinal peptide, two receptor-based radioligands, have been used to localize primary endocrine tumors as well as metastaticliver lesions. See A. Kurtaran, et al., xe2x80x9cVasoactive Intestinal Peptide and Somatostatin Receptor Scintigraphy for Differential Diagnosis of Hepatic Carcinoid Metastasis,xe2x80x9d The Journal of Nuclear Medicine, vol. 38, pp. 880-881 (1997).
An ideal radioligand therapy agent would accumulate selectively in target cells. The effectiveness of radiotherapy is due to the destruction of dividing cells resulting from radiation-induced damage to cellular DNA. See W. D. Bloomer et al., xe2x80x9cTherapeutic Application of Iodine-125 Labeled Iododeoxyuridine in an Early Ascites Tumour Model,xe2x80x9d Current Topics in Radiation Research Quarterly, vol. 12, pp. 513-25 (1977). In both therapeutic and imaging applications, any unbound, circulating radioligand is rapidly cleared by excretory systems, which helps protect normal organs and tissues. The radioligand may also be degraded by body processes which will increase the clearance of the free radioisotope. See G. A. Wiseman et al., xe2x80x9cTherapy of Neuroendocrine Tumors with Radiolabelled MIBG and Somatostatin Analogues, xe2x80x9d Seminars in Nuclear Medicine, vol. XXV, No. 3, pp. 272-278 (1995).
In both tumor imaging and therapy, a clinical goal is to maximize the amount of radiolabeled compound taken up by the tumor. The amount of radidligand that accumulates in target cells depends on many factors, for example: (1) the concentration gradient of the radioligand between the blood and the targeted tissue; (2) the number of cellular receptors, membrane or intracellular, and the affinity of those receptors for the radioligand; (3) the relative concentrations of labeled and unlabeled ligand competing for a given receptor; (4) the recycling rate for the cellular receptors; (5) the capacity of the cell to store the radioligand; and (6) the degradation of the radioligaud inside the cell. See R. K. Rippley et al., xe2x80x9cEffects of Cellular Pharmacology on Drug Distribution in Tissues,xe2x80x9d Biophysical Journal, vol. 69, pp. 825-839 (1995).
Radiolabeled compounds have typically been administered by intravenous, bolus injection. In a few instances, radiolabeled compounds have been given as infusions over 30 to 60 min, usually to limit side effects of the drug, not to increase efficacy. See e.g., H. P. Kalofonos et al., xe2x80x9cAntibody Guided Diagnosis and Therapy of Brain Gliomas using Radiolabeled Monoclonal Antibodies Against Epidermal Growth Factor Receptor and Placental Alkaline Phosphatase,xe2x80x9d The Journal of Nuclear Medicine, vol. 30, pp. 163-645 (1989); I. Virgolini et al., xe2x80x9cVasoactive Intestinal Peptide-Receptor Imaging for the Localization of Intestinal Adenocarcinomas and Endocrine Tumors,xe2x80x9d The New England Journal of Medicine, vol. 331, pp., 1116-21 (1994); G. A. Wiseman et al., xe2x80x9cTherapy of Neuroendocrine Tumors with Radiolabelled MIBG and Somatostatin Analogues,xe2x80x9d Seminars in Nuclear Medicine, vol. XXV, no. 3, pp. 272-78 (1995); S. W. J. Lamberts et al., xe2x80x9cSomatostatin-Receptor Imaging in the Localization of Endocrine Tumors,xe2x80x9d The New England Journal of Medicine, vol. 323, pp. 126-49 (1990); E. P. Krenning et al., xe2x80x9cSomatostatin Receptor Scintigraphy with Indium-111-DTPA-D-Phe-1-Octreotide in Man: Metabolism, Dosimetry and Comparison with Iodine-123-Tyr-3-Octreotide,xe2x80x9d The Journal of Nuclear Medicine, vol. 33, pp. 652-58 (1992); E. P. Krenning et al., xe2x80x9cLocalisation of Endocrine-Related Tumours with Radioiodinated Analogue of Somatostatin,xe2x80x9d The Lancet, vol. 1989, no. 1, pp. 242-244 (1989). There is one report of an infusion duration of two (2) hours. See J. A. Carrasquillo et al., xe2x80x9cIndium-111 T101 Monoclonal Antibody is Superior to Iodine-131 T101 in Imaging of Cutaneous T-Cell Lymphoma,xe2x80x9d The Journal of Nuclear Medicine, vol. 28, pp. 281-87 (1987).
The ability of a cell to take up a radiolabeled compound in the short term is limited by the number of cellular receptors or transport proteins for the compound on the cell membrane or within the cell. When the radioligand is administered by bolus injection, the binding pharmocokinetics dictate that uptake of the radioligand is linearly related to the amount injected only at low concentrations of the radioligand. At higher concentrations, the receptors for the radioligand become saturated. See H. Zhu et al., xe2x80x9cPotential and Limitations of Radioimmunodetection and Radioimmunotherapy with Monoclonal Antibodies,xe2x80x9d The Journal of Nuclear Medicine, vol. 38, no. 5, pp. 731-41 (1997); and R. M. Kessler et al., xe2x80x9cHigh Affinity Dopamine D2 Receptor Radioligands. 1. Regional Rat Brain Distribution of Iodinated Benzamides,xe2x80x9d The Journal of Nuclear Medicine, vol. 32, pp. 1593-1600 (1991). These saturated receptors are not able to bind more radioligand until either the receptor releases the radioligand, or the receptor-radioligand complex has been transported to another part of the cell and the receptor has been recycled to again bind a new molecule of the radioligand. Because the circulating unbound radioligand is rapidly eliminated, by the time the receptors are free to accept another molecule of the radioligand, the radioligand may no longer be present. Thus, the accumulation of radioligand depends on the availability of unbound radioligand, and on the recycling time of the cellular receptors and transport proteins.
The recycling of the cellular receptors depends on the fate of the ligand-receptor complex. Many, if not most, peptide compounds (including peptide and protein hormones) that bind to surface receptors are internalized as a ligand-receptor complex by endocytosis, i.e., invagination of the plasma membrane. Examples of peptides that have been shown to be internalized as part of a ligand-receptor complex include nerve growth factor, fibroblast growth factor, epidermal growth factor, platelet-derived growth factor, cholecystokinin, vascular endothelial growth factor, vasoactive intestinal peptide, gastrin-releasing peptide, leukemia inhibitory factor, somatostatin, oxytocin, bombesin, calcitonin, arginine vasopressin, angiotensin II, atrial natriuretic peptide, insulin, glucagon, prolactin, growth hormone, gonadotropin, thyrotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, corticotropin-releasing hormone, interleukins, interferons, transferrin, substance P, neuromedin, neurotensin, neuropeptide Y, and various opioids. This internalization takes timexe2x80x94minutes or even hours. See G. Morel, xe2x80x9cInternalization and Nuclear Localization of Peptide Hormones,xe2x80x9d Biochemical Pharmacology, vol. 47(1), pp. 63-76 (1994); D. Nouel et al., xe2x80x9cDifferential Internalization of Somatostatin in COS-7 Cells Transfected with SST1 and SST2 Receptor Subtypes: A Confocal Microscopic Study Using Novel Fluorescent Somatostatin Derivatives,xe2x80x9d Endocrinology, vol. 138, pp. 296-306 (1997); L.-H. Wang et al., xe2x80x9cLigand Binding, Internalization, Degradation and Regulation by Guanine Nucleotides of Bombesin Receptor Subtypes: A Comparative Study,xe2x80x9d Biochimica et Biophysica Acta, vol. 1175, pp. 232-242 (1993). Even monoclonal antibodies have been shown to be internalized into the cell. See O. W. Press et al., xe2x80x9cComparative Metabolism and Retention of Iodine-125, Yttrium-90, and Indium-111 Radioimmunoconjugates by Cancer Cells,xe2x80x9d Cancer Research, vol. 56, pp. 2123-29 (1996).
After internalization, many peptides translocate into the nucleus and even bind DNA. Peptides that been shown to accumulate in the nuclei of target cells include insulin, growth hormone, prolactin, nerve growth factor, somatostatin, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, and interferons. Nuclear binding sites have been described for gonadotropin-releasing hormone, gonadotropin, growth hormone, angiotensin II, prolactin, transferrin, insulin, various interleukins, glucagon, various opioids, and growth factors (including epidermal growth factor, nerve growth factor, platelet-derived growth factor and fibroblast growth factor). Insulin and epidermal growth factor have been shown to cause specific nuclear effects. See G. Morel, xe2x80x9cInternalization and Nuclear Localization of Peptide Hormones,xe2x80x9d Biochemical Pharmacology, vol. 47(1), pp. 63-76 (1994); and P. M. Laduron, xe2x80x9cFrom Receptor Internalization to Nuclear Translocationxe2x80x94New Targets for LongTerm Pharmacology,xe2x80x9d Biochemical Pharmacology, vol. 47, pp. 3-13 (1994).
Steroid hormones are known to diffuse through the plasma membrane and then either bind intracellular receptors and translocate to the nucleus, or directly bind receptors in the nucleus. See W. V. Welshons et al., xe2x80x9cNuclear Localization of Unoccupied Oestrogen Receptors,xe2x80x9d Nature, vol. 307, pp. 747-49 (1984). Classes of steroid hormones known to bind to intracellular receptors include progestins (e.g., progesterone), androgens (e.g., testosterone), glucocorticosteriods (e.g., hydrocortisone), mineralocorticoids (e.g., aldosterone), and estrogens (e.g., estradiol). See D. J. Sutherland et al., xe2x80x9cHorrnones and Cancer,xe2x80x9d The Basic Science of Oncology, 2d Ed. (I. F. Tannock and R. P. Hill, eds.), Chapter 13, pp. 207-231 (1992). Breast and prostate tumor cells are known to possess increased numbers of steroid hormone receptors.
One method that has been used to increase the tumor-to-background ratio of radioligand for therapy or imaging is to decrease the uptake of radioactivity by the background tissue by altering the rapidity of degradation or excretion. When the background radiation level decreases, the tumor-to-background ratio increases; however, the amount of radioligand accumulated by the tumor cell remains the same. Thus the actual therapeutic dose (the dose inside the cell) does not change, even though the tumor image will show more contrast against the background.
Methods that have been used to increase tumor cell uptake of the radioligand, and thus increase the therapeutic or diagnostic dose, include the following: using a radioligand more targeted to the tumor cells, using a radioligand with a higher diffusion rate into; the tissue, changing the elimination rate of the radioligand using a radioligand with a longer biologic half-life, using a radioisotope with a longer physical half-life, and using a higher dose of the radioligand; The radioligand has been administered either by a single bolus dose or by short infusion of up to 2 hours. Models have been developed to try to identify parameters that can be optimized to make the uptake more efficient. These models share the basic assumption that the radiolabeled compound is given in a single bolus dose. See Rippley et al. (1995); S.-E. Strand et al., xe2x80x9cPharmacokinetic Modeling,xe2x80x9d Medical Physics, vol. 20(2), Pt. 2, pp. 515-27 (1993); and H. Zhu etal., xe2x80x9cPotential and Limitations of Radioimmunodetection and Radioimmunotherapy with Monoclonal Antibodies,xe2x80x9d The Journal of Nuclear Medicine,. vol. 38, no. 5, pp. 731-41 (1997). There is a need for a method to increase the accumulation of the radioligand by the target cells without an increase in destruction of normal cells. Radiolabeled analogs of the peptide somatostatin have been studied for their effectiveness in tumor imaging and therapy. See E. A. Woltering et al., xe2x80x9cThe Role of Radiolabelled Somatostatin Analogs in the Management of Cancer Patients,xe2x80x9d Principles and Practice of Oncology, Vol. 9, pp. 1-15 (1995); U.S. Pat. No. 5,590,656; and U.S. Pat. No. 5,597,894. Endogenously produced somatostatin, a tetradecapeptide, inhibits release of several pituitary and intestinal factors that regulate cell proliferation, cell motility, or cellular secretion, including growth hormone, adrenocorticotropin hormone, prolactin, thyroid stimulating hormone, insulin, glucagon, motilin, gastric inhibitory peptide (GIP), vasoactive intestinal peptide (VIP), secretin, cholecystokinin, bombesin, gastrin releasing peptide (GRP), gastrin adrenocorticotropic hormone (ACTH), thyroid releasing hormone (TRH), cholecystokinin (CCK), aldosterone, pancreatic polypeptide (PP), various cytokines (e.g., interleukins, interferons), various growth factors (e.g., epidermal growth factor, nerve growth factor), and various vasoactive amines (e.g., serotonin).
Because somatostatin has a short biologic half-life (1 to 2 min), a variety of somatostatin peptide analogs have been produced by elimination of amino acids, by substitution of native L-amino acids with the corresponding D-amino acid isomers, by addition of an alcohol to the carboxy terminus of the molecule, or by various combinations of these approaches. See U.S. Pat. No. 5,597,894. Examples of somatostatin analogs include octreotide acetate, lanreotide, vapreotide (xe2x80x9cRC-160xe2x80x9d), and pentetreotide, all which have a longer biologic half-life. Multi-tyrosinated somatostatin analogues have been produced and shown to bind somatostatin cellular receptors. See U.S. Pat. No. 5,597,894.
Somatostatin receptors are found throughout the cell, including the cell membrane, Golgi apparatus, endoplasmic reticulum, vesicles, and nucleus. Somatostatin and its analogs are internalized by endocytosis of the ligand-receptor complex. See L. J. Hofland et al., xe2x80x9cInternalization of the Radioiodinated Somatostatin Analog [125I-Tyr3] Octreotide by Mouse and Human Pituitary Tumor Cells: Increase by Unlabeled Octreotide,xe2x80x9d Endocrinology, vol. 136, pp. 3698-3706 (1995); Wiseman et al., (1995).
High densities of somatostatin receptors, especially somatostatin receptor subtype 2 (SST-2), have been found on cells from a wide variety of tumors, including endocrine tumors, melanomas, breast carcinomas, Merkel cell tumors, lymphomas, small cell lung carcinomas, gastrointestinal tumors, astrocytomas, gliomas, meningiomas, carcinoid tumors, islet cell tumors, renal cell carcinomas, neuroblastomas, and pheochromocytomas. See E. A. Woltering et al., xe2x80x9cThe Role of Radiolabeled Somatostatin Analogs in the Management of Cancer Patients;xe2x80x9d Principles and Practice of Oncology, Vol. 9, pp. 1-15 (1995); and E. A. Woltering et al., xe2x80x9cSomatostatin Analogs: Angiogenesis Inhibitors with Novel Mechanisms of Action,xe2x80x9d Investigational New Drugs, vol. 15, pp. 77-86 (1997). The radiolabeled somatostatin analog 111In-Pentetreotide, known to bind SST-2 receptors on cell membranes, has been shown to bind to pituitary tumors, endocrine pancreatic tumors, carcinoids, paragangliomas, pheochromocytomas, medullary thyroid carcinomas, small-cell-lung cancers, neuroblastomas, meningiomas, breast carcinomas, renal cell carcinomas, gliomas, astrocytomas, melanomas, and lymphomas. 111In-Pentetreotide has also been used to treat metastatic glucagonoma and carcinoid tumors. See Wiseman et al., 1995; Krenning et al., xe2x80x9cRadiotherapy with a radiolabelled somatostatin analogue, [111In-DTPA-D-Phe1]-octreotide. A Case History,xe2x80x9d Annals of the New York Academy of Sciences, vol. 733, pp. 496-506 (1996); and M. Fjalling et al., xe2x80x9cSystemic radionuclide therapy using indium-111-DTPA-D-Phe-1-octreotide in midgut carcinoid syndrome,xe2x80x9d Journal of Nuclear Medicine, vol. 37, pp. 1519-21 (1996).
Radiolabeled somatostatin or somatostatin analogs have been used for tumor imaging and therapy, but have previously been administered either by bolus injection or by short infusion (up to 2 hours). See S. W. J. Lamberts et al., xe2x80x9cSomatostatin-Receptor Imaging in the Localization of Endocrine Tumors,xe2x80x9d The New England Journal of Medicine, vol. 323, pp. 124-649 (1990); E. P. Krenning et al., xe2x80x9cSomatostatin Receptor Scintigraphy with Indium-111-DTPA-D-Phe-1-Octreotide in Man: Metabolism, Dosimetry and Comparison with Iodine-123-Tyr-3-Octreotide,xe2x80x9d The Journal of Nuclear Medicine, vol. 33, pp. 652-58 (1992); E. P. Krenning et al., xe2x80x9cLocalisation of Endocrine-Related Tumours with Radioiodinated Analogue of Somatostatin,xe2x80x9d The Lancet, vol. 1989, no. 1, pp. 242-244 (1989); W. A. P. Breeman et al., xe2x80x9cStudies on Radiolabelled Somatostatin Analogues in Rats and in Patients,xe2x80x9d The Quarterly Journal of Nuclear Medicine, vol. 40, pp. 209-220 (1996); and E. P. Krenning et al., xe2x80x9cSomatostatin Receptor Scintigraphy with [111In-DTPA-D-Phe1]- and [123I-Tyr3]-octreotide: the Rotterdam Experience with More than 1000 Patients,xe2x80x9d European Journal of Nuclear Medicine, vol. 20, pp. 716-31 (1993).
Radiolabeled somatostatin analogs that have been used for tumor imaging or therapy include 111In-pentetreotide ((111In-DTPA-D-Phe1)-octreotide), (111In-DOTA0-D-Phe1-Tyr3)-octreotide, (90Y-DOTA0-D-Phe1-Tyr3)-octreotide, (86Y-DOTA0-D-Phe1-Tyr3)-octreotide, (111In-DTPA-D-Phe1)-RC-160, 99mTc-RC-160, 99mTc-octreotide, 188Re-RC-160, 123I-tyr3-octreotide, 125I-tyr3-octreotide, 125I-lanreotide, 90Y-DOTA-lanreotide, and 131I-WOC-3. See, e.g., Woltering et al., 1995; M. L. Thakur et al., xe2x80x9cRadiolabeled Somatostatin Analogs in Prostate Cancer,xe2x80x9d Nuclear Medicine and Biology, Vol. 24, pp. 105-113 (1997); M. de Jong et al., xe2x80x9cYttrium-90 and Indium-111 Labelling, Receptor Binding and Biodistribution of [DOTA0, D-Phe1, Tyr3]octreotide, a Promising Somatostatin Analogue for Radionuclide Therapy,xe2x80x9d European Journal of Nuclear Medicine, vol. 24, pp. 368-371 (1997); W. A. P. Breeman et al., xe2x80x9cA New Radiolabelled Somatostatin Analogue [111In-DTPA-D-Phe1]RC-160: Preparation, Biological Activity, Receptor Scintigraphy in Rats and Comparison with [111In-DTPA-D-Phe1]octreotide,xe2x80x9d European Journal of Nuclear Medicine, vol. 21, no. 4, pp. 323-335 (1994); W. A. P. Breeman et al., xe2x80x9cStudies on Radiolabeled Somatostatin Analogues in Rats and in Patients,xe2x80x9d The Quarterly Journal of Nuclear Medicine, vol. 40, no. 3, pp. 209-219 (1996); M. Fjalling et al., xe2x80x9cSystemic Radionuclide Therapy Using Indium-111-DTPA-D-Phe1-Octreotide in Midgut Carcinoid Syndrome,xe2x80x9d Journal of Nuclear Medicine, vol. 37, pp. 1519-1521 (1996); H. Kolan et al., xe2x80x9cSandostatin(copyright) Labeled with 99mTc: In Vitro Stability, In Vivo Validity and Comparison with 111In-DTPA-Octreotide,xe2x80x9d Peptide Research, vol. 9, no. 3, pp. 144-150 (1996); I. Virgolini et al.,xe2x80x9cxe2x80x98MAURITIUSxe2x80x99: Biodistribution, Safety and Tumor Dose in Patients Evaluated for Somatostatin Receptor-Mediated Radiotherapy,xe2x80x9d Paper Submitted to Journal of Nuclear Medicine (1997); and U.S. Pat. No. 5,597,894.
Somatostatin analogs have also been demonstrated to inhibit angiogenesis in tumors. A primary tumor initiates neovascularization by angiogenic stimulation. See M. S. O""Reilly, xe2x80x9cAngiostatin: An Endogenous Inhibitor of Angiogenesis and of Tumor Growth,xe2x80x9d in I. Goldberg et al. (eds.), Regulation of Angiogenesis, pp. 273-294 (1997). The growth of a solid tumor is dependent on neovascularization. This angiogenic tissue has been shown to be rich in somatostatin receptors subtype 2 (SST-2), and to be inhibited by somatostatin analogs known to bind SST-2 receptors, e.g., octreotide acetate, RC-160, and lanreotide. See E. A. Woltering et al., xe2x80x9cThe Role of Radiolabeled Somatostatin Analogs in the Management of Cancer Patients, xe2x80x9d Principles and Practice of Oncology, Vol. 9, pp. 1-15 (1995); E. A. Woltering et al., xe2x80x9cSomatostatin Analogs: Angiogenesis Inhibitors with Novel Mechanisms of Action,xe2x80x9d Investigational New Drugs, vol. 15, pp. 77-86 (1997); P. C. Patel et al., xe2x80x9cPostreceptor Signal Transduction Mechanisms Involved in Octreotide-Induced Inhibition of Angiogenesis,xe2x80x9d Surgery, vol. 116, pp. 1148-52 (1994); R. Barrie et al., xe2x80x9cInhibition of Angiogenesis by Somatostatin and Somatostatin-like Compounds Is Structurally Dependent,xe2x80x9d Journal of Surgical Research, vol. 55, pp. 446-450 (1993); and E. A. Woltering et al., xe2x80x9cSomatostatin Analogues Inhibit Angiogenesis in the Chick Chorioallantoic Membrane,xe2x80x9d Journal of Surgical Research, vol. 50, pp. 245-251 (1991).
Angiogenic blood vessels have SST-2 receptors at a higher density than vessels from normal tissues. See J. C. Watson et al., xe2x80x9cUp-Regulation of Somatostatin Receptor Subtype 2 (SST-2) mRNA Occurs During the Transformation of Human Endothelium to the Angiogenic Phenotype,xe2x80x9d Paper Presented at the 12th International Symposium on Regulatory Peptides, Copenhagen, Denmark, September 1996; and J. C. Watson et al., xe2x80x9cSST-2 Gene Expression Appears During Human Angiogenesis,xe2x80x9d Regulatory Peptides, vol. 64, p. 206 (Abstract) (1996). Radiolabeled somatostatin analogs binding to SST-2 receptors on tumor vessels have been used for radioimaging and radiotherapy. The tumor""s size deceases because blood vessel growth is inhibited by the radiolabeled compound. See J. C. Reubi er al., xe2x80x9cHigh Density of Somatostatin Receptors in Veins Surrounding Human Cancer Tissue: Role in Tumor-Host Interaction?,xe2x80x9d International Journal of Cancer, vol. 56, pp. 681-88 (1994). Pathologic blood vessel growth has also been implicated in several other disease conditions, including retinopathy of prematurity, diabetic retinopathy, glaucoma, tumor growth, rheumatoid arthritis, and inflammation. See Barrie et al., 1993. In fact, 111In-pentetreotide has been used to localize areas of joints affected by rheumatoid arthritis. See P. M. Vanhagen et al., xe2x80x9cSomatostatin Receptor Imaging: The Presence of Somatostatin Receptors in Rheumatoid Arthritis,xe2x80x9d Arthritis and Rheumatism, vol. 37, no. 10, pp. 1521-27 (1994).
Angiogenic cells have also been shown to express the vascular endothelial growth factor (VEGF) receptor gene, kdr, while quiescent vascular cells did not express this receptor gene. Both angiogenic and quiescent cells expressed the VEGF receptor gene, flt-1. See J. C. Watson et al., xe2x80x9cInitiation of kdr Gene Transcription is Associated with Conversion of Human Vascular Endothelium to an Angiogenic Phenotype,xe2x80x9d Surgical Forum, vol. 47, pp. 462-64 (1996).
Non-radiolabeled somatostatin analogs have been used to inhibit growth hormone secretion by infusion from an implantable pump system to avoid intermittent growth hormone release between injections, and to avoid the inconvenience to the patient of frequent subcutaneous injections. See G. Hildebrandt et al., xe2x80x9cResults of Continuous Long Term Intravenous Application of Octreotide via an Implantable Pump System in Acromegaly Resistent to Operative and X-ray Therapy,xe2x80x9d Acta Neurochirurgica, vol. 117, pp. 160-65 (1992). Thus the administration of the hormone by infusion allowed the concentration of the hormone to remain at a constant level in the blood stream. The objective was not to increase accumulation of the hormone inside the cells.
Infusion has also been suggested for uptake of a radiolabeled pyrimidine analog for incorporation into the DNA during DNA synthesis when the cell is in a growth phase. Pyrimidine does not bind to a cellular receptor, but instead is used as a building block for synthesizing new DNA. Because the radiolabeled pyrimidine in the general circulation was rapidly dehalogenated, infusion was suggested for increased uptake by solid tumors (e.g., breast cancer), which are known to have a slower growth rate and thus a lower percentage of cells in a growth phase at any one time than liquid tumors (e.g., leukemia). Infusion was suggested to maintain a level of radiolabeled compound in the circulation, so cells dividing at different cycles would have access to the radioligand for incorporation into DNA during DNA synthesis. No corresponding experimental data were given. No reference was made to radiolabeled compounds that bind to a cellular receptor. See Bloomer et al., 1977.
U.S. Pat. No. 5,590,656 discloses using a bolus injection of radiolabeled somatostatin to detect and differentiate neoplastic tissues.
U.S. Pat. No. 5,597,894 discloses using multi-tyrosinated somatostatin analogs given by bolus injection or short infusion (up to 60 min) to diagnose and treat tumors with peptide-specific surface receptors.
International Application (PCT) No. WO 91/01144 discloses using labeled polypeptide derivatives delivered by a single bolus injection or by a short infusion up to about 60 min for in vivo imaging of target tissues or therapy.
Radioimaging and radiotherapy are increasingly important in identifying and killing unwanted tumor cells. The effectiveness of the radioligand depends on the concentration that is accumulated in the target cells. Although methods have been developed to increase the tumor to background ratio, few methods have actually increased the concentration of the radioligand inside either the tumor cells or closely associated angiogenic cells. Thus, there is a need for a method to increase the accumulation and retention of radioligand inside the target cells without an increase in the destruction of normal body cells.
We have discovered that administering a radioisotopic compound by infusion over a period of time greater than two hours, preferably greater than twelve hours, greatly increases the maximum radioactivity that accumulates in the target cell. Accumulation of the radiolabeled compound in target tissues can be about five times higher than that resulting from bolus injection or short infusion methods. This method enhances the tumor-to-background radioactivity ratio by increasing the amount of radioligand accumulated inside the target cells. This method may be used with any radiolabeled compound whose cellular uptake rate is limited by binding to a cellular receptor or to a transport protein. Once the radiolabeled compound is internalized, the biological half-life plays no more than a minor role in the residence time. The primary factor governing residence time after internalization is the physical half-life of the radioisotope.